The objective of this research is to develop a digital-analog Computer Model of renal Homeostatic Mechanisms as modulated by Aging. Long-term goals include use of the model to originate or clarify hypotheses based on clinical observations and experimental renal physiology. Subjectively we all know that much of our physiology changes with age. However, this change is often difficult to document because physiological parameters that vary with age may not result in overt alteration of physiological variables regulated by homeostatic mechanisms. Negative feedback control systems are designed to regulate even when parts of the system are not functioning optimally. Digital-analog simulation has been used for a number of years as a formal means of developing hypotheses regarding the detailed functioning of physiological systems. Analog or mathematical models are particularly valuable in understanding temporal variations in physiological control systems that, in turn, help clarify cause and effect in complicated systems. By using models it will be possible to vary gains of various control loops and thus demonstrate the effect of minor changes such as might be brought about by aging on the dynamic response for the regulatory control mechanism. Furthermore, experiments using models may lead us to the choice of certain measurements that could be used to detect changes brought about by aging. After running a number of experiments on models that tend to differentiate between alternative hypotheses, the same experiments will be performed on animals and/or compared with clinical data.